In detection and ranging apparatuses, for instance in-vehicle radars (hereafter, a radar device is taken as a specific example of the detection and ranging apparatus), it is possible to reduce the size of the device, without sacrificing performance, by resorting to a technology wherein the aperture of a sensor array is enlarged through a combination of a transmission sensor array made up of a plurality of transmission sensor elements and a reception sensor array made up of a plurality of reception sensor elements. Such a technology is referred to, for instance, as holographic aperture synthesis, holograph or aperture synthesis. A radar device that utilizes aperture synthesis radiates probe signals from the plurality of transmission sensor elements (by time-division multiplexing, frequency division multiplexing, code division multiplexing or a combination thereof). The radar device receives then, at the plurality of reception sensor elements, echo signals generated through reflection of the probe signal in a target to be detected. The radar device synthesizes the signals received by the respective reception sensor elements to increase likewise the number of effective reception sensor elements, to increase the number of detectable targets, and to enhance angular resolution. Hereafter, a radar device that utilizes aperture synthesis will be referred to as synthetic aperture radar. Also, the sensor elements are used as antennas (elements), and the sensor array as an array antenna, without distinction.
[Patent document 1] Japanese Patent Application Publication No. 2000-155171
[Patent document 2] Japanese Patent Application Publication No. 2006-98181
[Patent document 3] Japanese Patent Application Publication No. 2007-155381
[Patent document 4] Japanese Patent Application Publication No. H11-231040
However, conventional synthetic aperture radars have a problem in that the target to be detected cannot be precisely detected under some specific conditions. Such conditions include, for instance, instances where a plurality of objects is present at virtually identical relative distances in the line of sight from the radar (hereinafter, referred to simply as “distances”), and instances where the objects move at substantially the same relative velocity (hereinafter, referred to simply as “velocity”) in the line of sight. A detailed examination of such cases reveals that there is a possibility that a target may fail to be detected accurately, also in daily recurrent scenes such as a traffic jam (i.e. where a plurality of objects moves with small velocity differences at close positions). Synthetic aperture radars, as in-vehicle radars, suffer from such problems.